Mastering is a term that's thrown around a lot but rarely understood. It's also the hardest to pull off at home. Mastering is the last stage in the recording process. Mastering takes all the separate songs for an album and puts them together so they sound good together. If mixed well, each song will sound good by itself, but that doesn't necessarily mean all the songs you put back to back on a CD will work together. Subtle differences in loudness and EQ from track to track can really hurt the sonic impact of a record. Mastering balances the sound from song to song so that the album sounds cohesive.
Mastering balances the levels from song to song, making sure that each song is as loud as it needs to be. It also makes sure that the tracks are relatively equal in volume and tone. Mastering might also apply some compression to smooth out the dynamics of the entire song. In many cases, some subtle EQ is applied to help polish up the tracks. Mastering deals only with the final stereo mix down, and it's not considered mixing anymore.
These days, mastering is done almost exclusively on the computer. There are many programs such as Wavelab and T-Racks mastering suite that let you work with just the stereo mix. You can also bring the final stereo file back into your audio program, such as Cubase or Pro Tools, to start mastering.
Compression and limiting are the most common processes applied during mastering. In order for a track to be heard well, it has to be loud enough. Even if you did a great job of setting levels in your recording, mastering sets the final loudness. Some compression might be applied to reduce the dynamic range of the audio. The negative effect of this compression is that you lose some of the volume of the track because the compression squeezes the sound together. Limiting then takes the compressed signal and boosts it to make tracks as loud as possible without clipping or going over the threshold you set. Once the track is compressed and boosted, it will start to “sit” better on the album. When this is repeated from track to track, you start getting something that sounds more like an album, rather than eight tracks thrown together on a CD.
Tone and Sequencing
When you put several songs together on an album, especially ones that were recorded over a long period of time, you might notice that the songs sound quite different from each other in regards to EQ and overall loudness. When you master, you can apply EQ to help the tonal balance of the songs fit together well. This typically means going though each song and listening for its EQ or looking at a graphic representation of the frequencies in your mastering program. Once all the tracks sound cohesive, the next step is deciding the sequence they should appear on the record. Unless you're recording a suite of songs that has a predetermined order, mastering is the stage when the order of songs occurs — this is called sequencing. Sometimes you can't help that certain songs sound different from one another, so in those cases careful sequencing can help them fit together on the album.
It's entirely possible for you to master at home, and you might even get good results. However, mastering might be the one part of recording you won't want to do at home. Mastering and a good mastering engineer are worth their weight in gold. Mastering really is an art — not to mention that mastering studios have hundreds of thousands of dollars of mastering equipment that can make your CD sound incredible.
If you've worked very hard at home, and you want to get the CD you've created put on the market, get it mastered by a professional. You'll be amazed at what a professional can do to a final mix.